Transistorized high-input-impedance amplifier



Oct. 29, 1968 G. P. WILSON 3,408,571

TRANSISTORIZED HIGH-INPUT-IMPEDANCE AMPLIFIER Filed Jan. 27, 1966MINIATURE l-TM. TRANSM/TTf/P 67 8 BATTER) SIG/ML aurPur GROUND I6 7INVENTOR.

GEORGEPAUL WILSON G IMM FM ATTORNEYS 3 Sheets-Sheet l Oct. 29, 1968 G P.WILSON 3,408,571 TRANSISTORIZED HIGH-INPUT-IMPEDANCE AMPLIFIER FiledJan. 27, 1966 3 Sheets-Sheet 2 8+ 22 VOLTAGE SUPPLY SIG/VAL 30U 7' PUT 0GROUND SIG/VAL OUTPUT GROUND INVENTOR. GEORGE PAUL WILSON ATTORNEYS Oct.29, 1968 G. P. WILSON 3,408,571

TRANSISTORIZED HIGHINPUTIMPEDANCE AMPLIFIER Filed Jan. 27, 1966 5shets-sheet 5 42 4 47 22 B- 40 VOLTAGE 45 SUPPLY 4s a v Y 4 37 3/ SIGNAL27 30 28 .34 33 ourPur GROUND E TRANSMITTER //V VE IV T 0/? 658965 PAULW/L 3011/ OQ U/MMMJ ATTORNEYS United States Patent Cfifice H 3,408,571Patented Oct. 29, 1968 3,408,571 TRANSISTORIZED HIGH-lNPUT-IMPEDANCEAMPLIFIER George Paul Wilson, San Anselmo, Calif. (5917 Fresno Ave.,Richmond, Calif. 94804) Continuation-impart of application Ser. No.330,116, Dec. 12, 1963. This application Jan. 27, 1966, Ser.

25 Claims. (Cl. 325145) ABSTRACT OF THE DISCLOSURE A self-containedminiaturized assembly for a microphone and a high-impedance-inputamplifier. A housing contains a condenser microphone, having first andsecond terminals, and an amplifier. The amplifier includes a fieldelfecttransistor having a gate connected to the second terminal and havingthird and fourth terminals, an amplifier transistor having a baseconnected to the third terminal, an emitter, and a collector, a D-Cpower supply connecting the signal output line to the ground line on thesignal output terminal side of the second resistor, and a fourthresistor and a first capacitor connected in parallel between the signaloutput side of the third resistor and the fourth terminal and thecollector in parallel. There may be a frequency-modulation transmitterin the same housing connected to the signal output line.

This application is a continuation-in-part of application Ser. No.330,116, filedDec. 12, 1963, now abandoned.

This invention relates to a high-input-impedance transistor amplifierfor use with a high-impedance transducer the conventional electron-tubecathode follower.

Proper operation of a condenser microphone or other the condenserelement of a condenser microphone, the electronic impedance-transformingdevice must be physically very close to the condenser element. Anysignificant the past this requirement has necessitated the placing of anelectron-tube cathode-follower physically at the end of the microphonecable along with the condenser element, as in U.S. Patent No. 2,579,162to Paul S. Veneprovided to supply the DC. currents necessary for properelectron-tube operation.

Attempts to use conventional transistors in the emitter follower orcommon collector configuration, have failed, even with so-calledbootstrapping, to provide a transistor device equivalent to theelectron-tube cathode follower, since conventional transistors provideinput impedance no greater than in the range of 10 to 10 ohms.

he object of the invention is to O ized impedance transforming devicethat can be used to megohms) or higher.

Another object of the invention is to lighten the weight and reduce thesize of the amplifier case for condenser microphones and similartransducers. The condenser microphone is, itself, very many times largerthan the microphone.

Condenser microphones have heretofore required a five-wire cable thatled from the electron-tube amplifier and was heavy and bulky,

reduce the power supply requirements of a condenser microphone assembly.The special power supply, formerly required, is eliminated, and a smallbattery can be used instead.

Another object of one form of the invention is to provide for the firsttime for wireless operation of a con- A further object is to provide animpedance transformer, for high-impedance transducers, which has lessground noise than conventional electron-tube devices.

The present invention provides a transistor amplifier input impedance.Simultaneously, the circuit can provide the polarizing charge or voltagenecessary to the operation of the condenser microphones.

The field-effect transistor differs from the conventional transistor inthat 1t uses and on both sides of it are N-type impurities forming P-Njunctions or gate regions. When a battery or power supply is connectedacross the ends of the P-type chan- The gate voltage thus controls theeffective area of the P-type bar or channel and therefore controls theoutput current of the device. In the other type offield-effect-transistor, the bar or channel is of the N-type materialand the gate is of P-type material; similar results are obtained. Sincethe only current required to control the output current of either typeof field-effect transistor is that which leaks through thereverse-biased gate to the channel junction, a high input resistance isobtained, and the device is almost a voltagedriven active circuitelement. I explain below how my circuit takes advantage of theproperties of field-effect transistors. It should be noted, however,that conventional field-effect transistor circuits give input impedancesseveral orders of magnitude less than my new circuit.

The invention is, in general terms, an impedance transforming device foruse with transducers requiring extremely high input impedanceamplifiers. While primarily the device is intended for use withcondenser (capacitor) type transducers, other applications of thetransistor circuit include use with crystal microphones, accelerometers,capacitor displacement gages, and, in short, any electrical device whichnormally requires a very high input impedance amplifier to provideproper operation. This invention provides input impedances of magnitudessimilar to those obtained with electron-tube cathode followers and muchgreater than can be obtained with conventional field-etfect-transistorcircuits or with an emitter-follower or other circuits using ordinarytwo-junction transistors. Moreover, simultaneously with providing highinput impedance, the polarizing charge (or polarizing voltage) forcondenser microphones provided by direct coupling.

Other objects and advantages of the invention will become apparent fromthe following detailed description of some preferred forms of theinvention. Details of the description and the drawings are furnished forpurposes of illustration of the invention and are not intended to implylimitation of the invention to these embodiments.

In the drawings:

FIG. 1 is a view in elevation of a condenser microphone system embodyingthe principles of the invention.

FIG. 2 is a similar view of a wireless" microphone system also embodyingthe principles of the invention.

FIG. 3 is an electrical circuit diagram of one form of my new amplifierconnected to a condenser microphone and employing a field-effecttransistor with a P-type channel.

FIG. 4 is a view similar to FIG. 3 of a circuit using a field-effecttransistor with an N-type channel.

FIG. 5 is an electrical circuit of a more elaborate form of my newcircuit which provides input impedance much greater than ohms, i.e., 210 ohms, using a P- channel field-effect transistor.

FIG. 6 is a circuit diagram generally similar to FIG. 5 for a deviceusing an N-channel field-effect transistor.

FIG. 7 is a diagram like FIG. 3 of a modified form thereof.

FIG. 8 is a circuit like FIG. 5 showing the use of a Zener diode as abiasing element.

A condenser-microphone assembly 10 is shown in FIG. 1 at substantiallyfull scale. A small electrically conductive tubular housing 11 isemployed; it may be, as shown, rather hour-glass shape with a narrowneck 12 between two enlarged ends 13 and 14, or, if desired, it may becylindrical. A condenser microphone 15 is carried in one end 13, and myamplifier is within the housing 11, including a field-effect transistor17 close to the microphone 15 and an amplifier transistor 18 in the end14. A three-terminal plug 19 is provided at the end 14 for connection toa light-weight three-wire cable 20 having a three-terminal socket 21.The plug 19 has terminals 22, 23 and 24.

The microphone 15 is preferably a small capacitor microphone of the typeused by the broadcast, movie, recording, and entertainment industriesand also used for precision acoustical measurements of all types. Thesemicrophones have capacitances in the range of 5 micromicrofarads to 100micro-microfarads, requiring amplifier input impedances in the range of1000 megohms and higher to maintain uniform or fiat frequency responsefor low audio frequencies. Acoustic measurements typically requireuniform response to frequencies as low as the 20 to 30 cycle per secondregion.

The unit 10 provides a physical arrangement which satisfies the need forlocating the impedance transforming device physically very close to thetransducer 15, a requirement which must be met whether the impedancetransforming device is an electrontube or a solid-state device.

To generate a signal voltage due to applied sound pressure (or othermeans of moving one of the transducer plates) the transducer capacitor15 needs a charge of electrons which remains constant. This charge isgenerally obtained by applying a high DC. voltage, in the range of 50 to400 volts, to the terminals of the transducer. The impedance of this DC.voltage source must be very high in order to prevent the charge fromleaking away as the voltage across the transducer terminals changes dueto the mechanical motion of the movable plate 39. In this invention, thecharge is provided through a resistor 25, as shown in FIG. 3, whichtypically has a value of 10 megohms (or a value in the range of 2 to 25megohms); the circuit of this invention is so arranged that theeffective value of the resistor 25 can be more than 2000 times itsactual value.

The circuit of FIG. 3 shows the amplifier that is within the housing 11of FIG. 1. The plug prong or terminal 22 is connected to a B- voltagesupply such as the negative terminal of a battery and the prong orterminal 24 is grounded, as through the case 11 and to the positiveterminal of a battery. The signal output goes to the prong or terminal23. One terminal of the condenser microphone 15 is connected by a lead26 to the ground terminal 24, and the other terminal is connected bywire leads 27 and 28 to the resistor 25 and by leads 27 and 29 to thegate 30 of the fieldetfect transistor 17, which may typically be a2N2498. The resistor 25 may be connected by a lead 31 to the signaloutput terminal 23, and a very important resistor 32 is connected acrossthe leads 26 and 31. A typical value of the resistor 32 is 100,000 ohms.(FIG. 7 shows a circuit like FIG. 3 but with the signal output 23 beingconnected to the circuit by a lead 31a between the resistor 35 and thecollector 38. There is no substantial difference in operation betweenFIGS. 3 and 7. In other words the signal output 23 may be connected toeither end of the resistor 35. FIG. 7 also shows that with nonpolarizedtypes of transducers, the transducer is coupled through couplingcapacitor 71.)

A lead 33 connects a terminal 34 of the field-effect transistor 17 tothe resistor 32 through a resistor 35, having a typical value of 2,000ohms. A condenser 36, typically microfarad capacity, bridges the leads33 and 31.

The resistor 35 is also connected by a lead 37 to the collector 38 ofthe transistor 18, which may typically be a 2N388A, while the terminal40 of the field-effect transistor 17 is connected to the base 41 of thetransistor 18 by leads 42 and 43. A resistor 44 is connected to thejunction of the leads 42 and 43, and the resistor 44 and a lead 45 fromthe emitter 46 of the transistor 18 are connected to a voltage supplylead 47 that goes to the voltage supply terminal 22.

The invention thus uses a specially connected fieldeffect transistor 17to provide high input impedance as necessary for use with the condensermicrophone 15 or other high impedance transducers. The special circuitprovides extremely high input impedance, accomplishes impedancetransformation, and simultaneously provides the necessary polarizingvoltage for the capacitor microphone 15. The special and unusualproperties of the using the common electrode configuration of thetransistor 18 and the field-effect transistor 17, and by using arelatively high power-supply voltage. The value of resistor 32 lies inthe range of 1000 ohms to megohms, and the The value of the resistor 44(typically 500 ohms) depends ratio of the current in the field-efiectthat of the amplifying transistor 18. resistor 44 can be made,consistent with current requirements of the amplifying transistor 18,the higher will be the input impedance of the circuit.

The polarizating voltage for the condenser microphone is obtained as ofthe field-effect the gate bias resistor 25 to the high resistor 32. Bydirectly connecting one terminal of the capacitor microphone 15 to thegate terminal 30 of the field-effect transistor 17 and the terminals.

The circuit can be arranged to provide either a positive or a negativepolarizing voltage on the condenser microphone 15. By using theP-channel field etfect transistor 17 and an NBN-transistor 18, as inFIG. 3, a negative polarizing voltage for the microphone is obtained. Byusing an N-channel field-effect transistor 50 with a gate 52, a

' drain terminal 54) and a PNP- source, as shown in FIG. 4, for themicrophone 15 is similar results in other respects, and the circuitvalues may be the same.

The high input impedance is obtained largely by the common electrodeconfiguration and feed-back from the amplifying transistor 18 or 51 andpartially from the inherent high input impedance of the field-effecttransistor 17 or 50. The actual input impedance depends upon the gain ofthe amplifying transistor 18 (or 51), tude of the common resistor 32 andthe ratios of the resistances 32, 25, and 44. The resistance 35 servesonly the purpose of adjusting for proper operating voltage and currentsby adjusting the actual value of the field-effect transistor 17 (or 50)gate terminal DC. bias voltage. The to increase the feedback bybypassing changes in polarizing voltage.

adding a resistor 60 and capacitor 61, as shown in Y FIGS. 5 and 6 (or aZener diode 75, which is equivalent here, see FIG. 8), between hot line47, the value as: resistor 60, 3300 ohms, condenser 61, microfarads,resistor 44, 20,000 ohms, and resistor 35, 10,000 ohms,

20 and uses a standard three-pin microphone cable connector 19 and 21and therefore can be used with any standard dynamic microphone (or otherbalanced line) of both FIGS. 5 and 6 can be used without the F-Mtransmitter 67 and with a three-wire cable 20, and this, too, has itsadvantages.

The present invention has the interesting feature, when used withpolarization voltage, caused by charge leakage due to dampness, etc.,causes the field-effect transistor 17 to cease operating as an amplifierand to become effectively a Zener diode. The net eifect is that a changein polarization voltage of the microphone that is sulficient to cause asignificant change in the microphone sensitivity causes the microphonesignal to disappear completely and be replaced by a constant-amplituderushing noise, similar to the noise of a large waterfall. The presenceof this noise then indicates that the microphone needs attention, andthis indication prevents operation before any erroneous results can beobtained. This same effect makes the circuit self-protecting since theZener diode mode of operation of the field-effect transistor 17 preventsany voltage of too large magnitude from ever being impressed across thetransistors. Thus, a power supply voltage of twenty times the voltagerating of the transistors may be used, with no danger of burning out thetransistors 17 and 18, barring, of course, direct short circuits.

To those skilled in the art to which this invention relates; manychanges in construction and widely dilfering embodiments andapplications of the invention will suggest themselves without departingfrom the spirit and scope of the invention. The disclosures and thedescription herein are purely illustrative and are not intended to be inany sense limiting.

I claim:

1. A transducer amplifier assembly for use with a highoutput-impedancetransducer having a first grounded terminal and a second terminal,comprising:

a field-effect transistor having a gate electrode connected to saidsecond terminal and having third and fourth terminals,

an amplifier transistor having a base connected to said third terminal,an emitter connected to said third terminal through a first resistor,and a collector,

means for connecting a power supply to said emitter,

a signal output line connected to said second terminal and said gateelectrode through a second resistor and connected to ground through athird resistor, the ungrounded side of said third resistor beingconnected to said fourth terminal and to said collector through a fourthresistor.

2. The assembly of claim 1 wherein there is a capacitor in parallel withsaid fourth resistor.

3. The assembly of claim 1 wherein said means for connecting the powersupply to the emitter, and the signal output line comprise a three-wirecable.

4. A high input impedance amplifier having a first grounded terminal anda second terminal, comprising:

a field-etfect transistor having a gate electrode connected to saidsecond terminal and having third and fourth terminals,

an amplifier transistor having a base connected to said third terminal,an emitter connected to said third terminal through a first resistor,and a collector,

means for connecting a power supply to said emitter,

a signal output line connected to said second terminal and said gateelectrode through a second resistor and connected to ground through athird resistor, the ungrounded side of said third resistor beingconnected to said fourth terminal and to said collector through a fourthresistor.

5. The amplifier of claim 4 wherein there is a capacitor in parallelwith said fourth resistor.

6. An amplifier for use with a condenser microphone having first andsecond terminals, said first terminal being grounded, comprising:

a field-effect transistor having a gate electrode connected to saidsecond terminal and having third and fourth terminals,

an amplifier transistor having a base connected to said third terminal,an emitter, and a collector,

a power supply connected to said emitter,

V a first resistor connecting said power supply to said third terminal,

a signal output line having a signal output terminal and connected tosaid second terminal through a second resistor,

a' third resistor connecting said signal output line to ground on thesignal output terminal side of said second resistor,

and a fourth resistor and a capacitor connected in parallel between saidsignal output line on the signal output terminal side of said secondresistor and said fourth terminal and said collector in parallel. 7. Theamplifier of claim 6 wherein said signal output line is connected to anF-M transmitter, with the entire assembly being a self-contained unitrequiring no wired connection outside of itself.

8. The amplifier of clzum 6 wherein a parallel resistance-capacitancenetwork connects said emitter to said power supply and to said firstresistor.

9. A miniaturized amplifier assembly for a transducer having a firstgrounded terminal and a second terminal,

a field-effect transistor having a gate electrode connected to saidsecond terminal and third and fourth terminals,

an amplifier transistor having a base connected to said third terminal,an emitter connected to said third terminal by a first resistor, and acollector,

a power supply terminal connected to said emitter,

a signal output line having a signal output terminal and connected tosaid second terminal through a second resistor,

a ground terminal,

a third resistor connecting said signal output terminal line to saidground terminal,

and a fourth resistor and a capacitor connected in parallel between (a)the signal output side of said third resistor and (b) said fourthterminal and said collector in parallel, whereby a simple three-terminalplug can connect said power supply terminal, said ground terminal, andsaid signal output terminal to a simple three-wire cable.

10. The amplifier assembly of claim 9 wherein a resistance-capacitancenetwork connects said emitter to said power supply terminal and to saidfirst resistor.

11. A self-contained miniaturized assembly for a microphone, comprising:

a housing having first and second ends,

a condenser microphone in said first end having first and secondterminals,

a field-effect transistor in said first end having a gate electrodeconnected to said second terminal and third and fourth terminals,

an amplifier transistor in said housing having a base connected to saidthird terminal, an emitter, and a collector,

a D-C power supply in said housing having a negative side and a positiveside connected to said first terminal through a ground line,

a first resistor in said first end connecting said negative side of saidpower supply to said third terminal,

a signal output line in said housing connected to said second terminalthrough a second resistor in said first end,

a third resistor in said housing connecting said signal output line tosaid ground line on the signal output terminal side of said secondresistor,

a fourth resistor and a first capacitor in said housing, connected inparallel between the signal output side of said third resistor and saidfourth terminal and said collector in parallel, and

a frequency-modulation transmitter in said housing connected to saidsignal output line.

'12. The assembly of claim 11 having a fifth resistor and a secondcapacitor in parallel in said housing connected between said emitter andsaid negative side of said power supply and the power supply side ofsaid first resistor.

I 13. A miniaturized amplifier assembly for an electrical device havinga first grounded terminal and a second terminal, f a a field-effecttransistor having a gate electrode connected to said second terminal andthird and fourth terminals,

second resistor, a ground terminal,

a third resistor connecting said second resistor to said groundterminal, and a fourth resistor and power supply terminal, said signaloutput terminal to a simple three-wire cable.

grounded terminal and a second terminal,

a field-effect transistor having a gate electrode connected to saidsecond terminal and third and fourth end of said third resistor.

16. The amplifier of claim 15 wherein there is a capacitor in parallelwith said third resistor.

17. The amplifier of claim 15 wherein a Zener diode, connects said powersupply and said first resistor to said emitter.

18. The amplifier of claim capacitance network connects said firstresistor to said emitter.

19. A microphone assembly, comprising:

ahousing,

a condenser microphone operated in the polarized mode in one end of saidhousing and having a first grounded terminal and a second terminal, and

an amplifier assembly comprising:

a field-efiect transistor having a gate electrode connected to saidsecond terminal and having third and fourth terminals,

an amplifier transistor having a base connected to said third terminal,an emitter connected to said third terminal through a first resistor,and a collector,

' means for connecting a power supply to said emitter,

a signal output line connected to said second terminal and said gateelectrode through a second resistor and connected to ground through athird resistor, the

15 wherein a resistancesaid power supply and ungrounded side of saidthird resistor being connected to said fourth terminal and to saidcollector through a fourth resistor,

whereby asimple three-terminal plug in the other end 'of said housingcan said 'ground terminal, and said signal output terminal to a simplethree-conductor cable.

20.'A self-contained miniaturized assembly for a microphone, comprising:

ahousing,

a condenser microphone in one end of said housing and having 'a firstgrounded terminal and second terminal,

'a field-effect transistor having a gate electrode connected to saidsecond terminal and third and fourth terminals, 1

an amplifier transistor having a base connected to said third terminal,a collector connected to said fourth and an emitter connected to saidthird terminal by a first resistor,

a power supply terminal connected tosaid emitter,

a second resistor connected to said second terminal and to saidcollector through a third resistor,

a fourth resistor connected from the junction of said second resistorand said third resistor to said first grounded terminal.

a signal output line and nected to said power supply terminal, and afrequency-modulation transmitter in said housing connected to saidsignal output line.

said means coupled between said gate electrode and said referencepotential including resistive means coupled between said referencepotential, said gate electrode and said electron collecting electrode.

22. In combination, a high input impedance semiconelcctrode, an electroncollecting electrode coupled to said source electrode and an electronemitting electrode,

providing a polanzation voltage for said transducer, and

feedback means responsive to signal current passing through said currentpath and said semiconductor device for feeding back substantially onehundred percent of the signal voltage to said gate electrode. 23. Incombination, a high input impedance semiconductor device having acurrent path defined by a source 1 1 electrode and a drain elect'rod'eand having a gate electrode responsive to an electric potential-thereonfor controlling the current in said current path, a semiconductor devicehaving a control electrode 'c oupledto said drain electrode, anelectroncollecting electrode coupled to said source electrode and anelectronemitting electrode, a

voltage source, coupled b tween said electron emitting electrode and areference potential, and feedback means responsive to signal currentpassing through said current path and said semiconductordevice forfeeding back substantially one hundred ,percent of .the outputsignalvoltage to said gate electrode. i

- 24. A combination as set forth in claim 23 wherein said feedback'meansincludes a firstl resistor. coupled between said reference potential andsaid electron collecting electrode and'a second resistor coupled betweensaid electron collecting electrode and said gate electrode. 7 1

' 25. A combination as set forth in'claim 23 further including a highimpedance variable capacitance transducer coupled between said referencepotential'and said gate electrode.

, s 12 r References Cited UNITED STATESPATENTS 3,300,585 1/1967 'Reedyk5 2,705,287 3/1955. Lo 225-105 2,771,584 11/1956 Thomas 325 -105.:2,899,549 *8/1959 POft61 325-361 3,082,380 3/1963 Herrmann 330-49 IOTHERREFERENCES v n 10 Bignelhflectronics, Mar. 8, 1963, pp. 4446,TK7800.

E58 33038FF.. Huang et al., Electronic Design, .Qctober 1955,. pp.42-45, T1 7300.E51330-38FE, e

Lylies, Electronic Design, Nov. 22,; l963,,pp. '71, 72, 15TK7800.E5133038FE. I

Product Survey-Field-Effect Transistor," Electronic Design, Apr.26,1963, pp. 6669, TK7800.E51 330- 38 FE. 6

20 ROBERT L. GRIFFIN, Primary lLra/ni ler I I A. I. MAYER, AssistantExaminer.

